It is common to process human speech signals to achieve a smaller bandwidth, thereby improving transmission efficiency. A key issue in such processing is achieving a lower signal bandwidth while maintaining acceptable speech quality. Low bit-rate encoders have been used to reduce the amount of voice signal information required for transmission or storage. In particular, linear predictive coding (hereinafter "LPC") encoders have been used in many low bit rate speech coding applications.
In a typical speech encoder the speech samples are blocked into 15 to 30 ms frames. Each frame may be further partitioned into N subframes, where N&gt;1. The frame of speech samples is parameterized by codes. Typically the speech spectral information is coded and transmitted at a frame rate, while other speech information may be coded and transmitted for each subframe. It is known that speech quality improvement may be achieved by updating the spectral parameters at the subframe rather than the frame rate, through interpolation. This process generally produces smoother sounding reconstructed speech, but at the expense of smearing the spectrum in the segments of speech where the speech spectrum changes rapidly.